Alkylbenzenes in diiodomethane. A novel, "primitive" micelle-forming surfactant system

We have investigated the extent to which alkylbenzenes exhibit surfactant p roperties in diiodomethane (DIM) as solvent. As a result of its dipole mome nt and high refractive index, DIM has a high cohesive energy density arisin g from dipole-dipole and dispersion forces. DIM has a high affinity for the benzene group of an alkylbenzene solute but low affinity for the alkyl cha in. These different affinities endow alkylbenzenes with amphiphilic propert ies in DIM solution, causing them to adsorb at the DIM-air surface and to a ggregate. Surface tensiometry measurements show that the adsorption of alky lbenzenes at the DIM-air surface increases with the alkylbenzene chain leng th, consistent with the notion that the antipathy between the DIM solvent a nd the alkyl chain of the alkylbenzene provides the main driving force for adsorption. Ellipsometry measurements confirm the adsorption of octyl- and hexylbenzenes at the DIM-air surface and also indicate multilayer adsorptio n of octylbenzene at high concentrations. Using dynamic light scattering it is shown that octylbenzene forms aggregates in DIM at concentrations highe r than 5 mol %. The apparent aggregate hydrodynamic radius increases from 0 .6 nm at low concentrations to 1.5 nm at high concentrations.